A fuel cell has been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and various other applications. In particular, the fuel cell has been identified as a potential alternative for the traditional internal-combustion engine used in modern vehicles.
One type of fuel cell is known as a proton exchange membrane (PEM) fuel cell. The PEM fuel cell typically includes three basic components: a cathode electrode, an anode electrode, and an electrolyte membrane. The cathode and anode electrodes typically include a finely divided catalyst, such as platinum, supported on carbon particles and mixed with an ionomer. The electrolyte membrane is sandwiched between the cathode and the anode to form a membrane-electrode-assembly (MEA). The MEA is often disposed between porous diffusion media (DM) which facilitate a delivery of gaseous reactants, typically hydrogen and oxygen, for an electrochemical fuel cell reaction.
Individual fuel cells can be stacked together in series to form a fuel cell stack. During start-up of the fuel cell stack, hydrogen gas is typically used to purge the anodes of air that diffuses into and accumulates on the anodes during shut-down. The flowing of hydrogen gas into the anodes after a shut-down creates a “hydrogen-air front” that travels across the anodes. The purge is desirably rapid to minimize the known carbon degradation that occurs as the hydrogen-air front moves across the anodes while air is on the cathodes. A conventional fuel cell system primarily employs the hydrogen gas pressure during the purge to displace the accumulated air. However, the rate of fill can be limited by pressure limitations of the fuel cell stack and flow resistances across the fuel cell system.
To mitigate carbon degradation, a short circuit of the fuel cell stack is sometimes performed during the purge. However, carbon corrosion may also be caused by a non-simultaneous delivery of hydrogen to the fuel cells. For example, the fuel cells nearest the hydrogen supply may receive hydrogen first, and the short circuit is not effective until most of the fuel cells have received hydrogen. Thus, the fuel cells that receive hydrogen first may experience unmitigated corrosion due to the hydrogen-air front. Additionally, when many of the fuel cells begin to receive hydrogen, the short circuit begins to operate. However, the fuel cells that do not have hydrogen may experience a negative voltage in a phenomenon known as “cell reversal.” Cell reversal also results in an undesirable carbon corrosion of the fuel cell stack.
Air is also bypassed to an exhaust of a fuel cell stack during start-up to dilute exhausted purge hydrogen. Vehicle emissions standards generally require the exhausted hydrogen concentration to be less than four percent (4%) by volume. However, due to the inconsistent conditions of the fuel cell system following a shut-down period, such as a variable quantity of accumulated air on the anodes, known fuel cell systems are not particularly effective in optimizing hydrogen emissions during start-up.
There is a continuing need for a fuel cell system and method that provide an efficient start-up while meeting desired hydrogen exhaust emissions standards. Desirably, the fuel cell system and methods provide a rapid system start-up with minimal stack degradation by optimizing the hydrogen-air front time during the start-up.